"We found that people tend to use the withdrawal method when they're not really planning ahead," Dr. Annie Dude, a resident in obstetrics and gynecology at the Duke University Medical Center in Durham, North Carolina, and lead author of the paper, said.

"It's a lot more common than many people realize," she told Reuters Health.

And, Dude added, her study shows withdrawal doesn't work as well as other birth control methods for avoiding unintended pregnancies.

The most effective birth control methods are longer-term reversible contraceptives, such as intrauterine devices (IUDs) and implants that go under the skin. Less than one woman in 100 will get pregnant each year using these forms of birth control.

Slightly less effective forms include the Depo-Provera hormone shot, the Pill, the ring and the patch, all with failure rates between two and nine percent per year.

When used properly, withdrawal carries about the same risk of pregnancy as condoms and diaphragms, with failure rates of 15 to 24 percent per year. But because withdrawal requires good timing and communication between partners, some experts estimate that failure rates may be even higher, between 18 and 28 percent.

Women who use the pull-out method instead of condoms are also at risk for sexually transmitted infections, such as Chlamydia and HIV.

For the new study, the researchers analyzed information from the National Survey of Family Growth, which contains data about the reproductive behavior of American women ages 15 to 44. Data were collected from 2006 to 2008 by interviewers and with self-administered computer surveys.

The researchers looked at the responses of 2,220 sexually active women between the ages of 15 and 24. They separated those women into two groups: women who had used withdrawal in at least one month of the study period, and women who only used other forms of birth control.

Just over one-quarter of women in the study said they had become pregnant during the prior 22 months, on average, and the majority of pregnancies - 59 percent - were unintended, according to findings published in Obstetrics and Gynecology.

Along with participants who used the pull-out method, African American and poor women were more likely than others to have had an unintended pregnancy. Compared to women who only used other forms of birth control, seven percent more withdrawal users had also taken emergency contraception.

"I'm really excited that the authors took a look at the use of withdrawal because it certainly reflects what I see in my own clinical practice," said Dr. Nancy Stanwood, section chief of family planning at the Yale University School of Medicine in New Haven, Connecticut.

Most unintended pregnancies happen among women between the ages of 15 and 24, so looking at the use of less effective birth control methods in this population is important, Stanwood told Reuters Health.

According to the Centers for Disease Control and Prevention, approximately 37 percent of U.S. births between 1982 and 2010 were unintended at the time of conception. Over three-quarters of births to women ages 15 to 20 were unintended and half of births to women ages 20 to 24 were unintended.

Having an unintended pregnancy is tied to poor prenatal care, which in turn is linked to birth complications including low birth weight, neonatal death and higher healthcare costs.

"The main point this study makes is that withdrawal as a form of contraception is more common than we thought," Stanwood said.

"It's associated with higher risk of unintended pregnancies, and higher risk of using emergency contraception. Women who use it might not recognize the degree of the risks they're taking."

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